SET(WIDGET_CALLBACK)

 Specifies the VAXTPU program or learn sequence to be called by VAXTPU when
 a widget callback occurs for the widget instance.

 Syntax

    SET (WIDGET_CALLBACK, widget, program_source, closure)

 Parameters

    WIDGET_CALLBACK      A keyword directing VAXTPU to set the
                         application-level widget callback.

    widget               The widget instance whose callback you want to
                         set.

    program_source       The string, buffer, range, learn sequence, or
                         program specifying the application-level callback.
                         This code is executed when the widget performs a
                         callback to VAXTPU.

    closure              A string or integer.  VAXTPU passes the value to
                         the application when the widget performs a
                         callback to VAXTPU.

 Example

    The following statement designates the procedure "user_scroll_dispatch"
    as the callback routine handling events from the scroll bar widget.
    The statement designates the closure for the callback as the character
    "h".

    SET (WIDGET_CALLBACK, scroll_bar_widget, "USER_SCROLL_DISPATCH", "h");